Enforce read-only status for virtual storages

Enforces storage's read-only mode for RPCs. This prevents a new primary from accepting writes after a failover before and administrator manually enables writes again for the storage. This prevents conflicts when the administrator would like to manually fix possible data loss on the new primary but the new primary has already updated some of the affected refs, causing conflicts.

The commit also adds 'praefect enable-writes' command that allows for enabling writes to a virtual storage again after a failover.

The functionality is behind 'enforce_read_only' feature flag and is switched off by default.

This depends on !2146 (merged), reviewing will be easier by looking at the last commit until it has been merged.

Closes #2630 (closed)